1. Field of the Invention
This invention relates to a method of fabricating a diffraction grating by using three hyperbolic diffraction gratings to record interference fringes on a photographic plate and to an apparatus for carrying out the method.
2. Prior Art Statement
A method has been reported for fabricating a diffraction grating by using a first diffraction grating, a second diffraction grating and a third diffraction grating to record interference fringes on a photographic plate. In the reported method the first diffraction grating (spatial frequency f/2) is illuminated with a quasi-monochromatic spherical wave to produce +1st and -1st order diffracted rays. The +1st order diffracted ray is -1st order diffracted by the second diffraction grating and the -1st diffracted ray is +1st order diffracted by the third diffraction grating and the resulting diffracted rays are merged on the photographic plate, where they produce interference fringes. The interference fringes are recorded on the photographic plate to obtain a diffraction grating. If all of the diffraction gratings are perfect gratings consisting of equally spaced straight grating lines, straight grating lines will be recorded on the photographic plate.
In actuality, however, it is difficult to obtain perfect gratings. If the grating lines are not linear and equally spaced in any of the diffraction gratings, the sums and differences of the such distortions will appear as distortion in the interference fringes recorded on the photographic plate. Generally speaking, the distortion in the recorded grating lines is greater than that in the diffraction gratings used to record them. This makes it difficult to fabricate a low-distortion grating.
The present invention was accomplished in the light of the foregoing circumstances and has as its object to provide a method and an-apparatus for fabricating a diffraction grating by using interference fringes produced by means of three diffraction gratings to record grating lines on a photographic plate, wherein the distortion of the recorded grating lines caused by any distortion of the grating lines of the three diffraction gratings is minimized by mutual cancellation.